Cyclic carbonates are used as organic solvents, agents f or processing synthetic fibers, raw materials for medicaments, cosmetic additives and electrolyte solvents for lithium batteries, and are also utilized for the synthesis of alkylene glycols and dialkyl carbonates, and the like (Patent Literature 1), and thus are one of important compounds that are used in a wide variety of applications.
Conventionally, the cyclic carbonates have been synthesized by reacting an epoxide and carbon dioxide in the presence of a homogeneous system catalyst under a suitable pressurized condition. As such homogeneous system catalyst, halides of alkali metals and onium salts such as quaternary ammonium salts have been conventionally known (Patent Literature 2), and are industrially used.
However, in the case when such homogeneous system catalyst is used, a separation operation to separate the reaction mixture and catalyst by distillation, or the like is generally required, and thus the production steps become complex, and there are problems of the decomposition of the catalyst during the separation step and the generation of by-products.
Thus, for the purpose of simplification of a separation process of a catalyst, a heterogeneous catalyst obtained by immobilizing a quaternary phosphonium having a halogenated ion as a counter ion onto a carrier such as silica gel has been suggested (Patent Literatures 3 and 4).
However, such a heterogeneous catalyst has a problem that an immobilized quaternary phosphonium salt gradually leaches during the reaction and an amount of a catalyst that relates to the reaction is substantially reduced, resulting in decreasing the yield over time. In particular, when the above described catalyst is used in a continuous process, frequent exchange of catalysts is required because of degradation of the catalysts and heterogeneous catalysts are thus not satisfactory enough from the viewpoints of production efficiency and costs of the catalysts. Therefore, it has been demanded to suppress degradation of a catalyst due to leaching out of a quaternary phosphonium salt and to keep catalytic activity over a long period of time.